Whether it’s “easy” and within your capabilities all depends what your capabilities and experience are! No insult intended - but unless you’re confident working with mains voltages I’d say get someone in. Also, think about the bit that says "Please remember that for your own safety Hive Active Heating should always be installed by a qualified professional.” and think about how that may impact your house insurance.
Having said all that....
Assuming it’s a combi boiler, you don’t have to worry about the hot water side, just the heating - a single channel controller. You join 2 terminals together, it comes on.
The manual above says that for a single channel controller, there are 6 terminals:
N: Permanent Neutral
L: Permanent Live
1: Common
2: Heating Off (NC)
3. Heating On (NO)
4. Unused
So there will be a changeover relay inside - when the heating is ON the “Normally Open (NO)” terminal is connected to Common.
So you run a 4 core +earth cable from boiler to time switch. The N and L get paralleled up with the incoming N and L feeds from the fused spur. That’s the permanent supply that powers the electronics.
You then find the 2 terminals on the boiler which are “join us to turn the boiler on” - they’ll probably be labelled “time switch” or something like that - and connect them to 1 (common) and 3 (Heating On - NO).
Disclaimer: the above is just the random thoughts of a bloke on the internet who you’ve never met, who’s qualifications you don’t know, and who has never seen your boiler. Treat it accordingly.